1. Field of the Invention
The present invention relates to an injection molding method of embedding a workpiece within a resin to form a molded article, and more particularly to an injection molding method for forming a component of an electrical device, for example, a base plate or cover plate of an electrical rotary switch for detecting positions of an automatic transmission of an automobile.
2. Disclosure of the Prior Art
In the past, injection molding has been utilized to integrally mold a workpiece within a resin to form a molded article. For example, a molded article 1M can formed by an injection molding method with the use of a mold 10M. The mold 10M comprises lower and upper dies 11M and 12M for making a mold cavity 13M therebetween, supporting pins 14M for supporting a workpiece 2M in the mold cavity, and a gate 15M for injecting a resin 3M into the mold cavity 13M, as shown in FIG. 12A. The supporting pins 14M are forced against the workpiece 2M to support the workpiece within the mold cavity 13M, and then the resin 3M is injected into the mold cavity 13M through the gate 15M, as shown in FIG. 12B. The resin 3M injected in the mold cavity 13M is hardened to obtain the molded article 1M, as shown in FIG. 12C.
In this molding method, since the molded article 1M is removed from the mold cavity 13M after the injected resin is hardened, there is a problem because holes 4M are left in the molded article 1M by removal of the supporting pins 14M from the molded article. In case of applying this molding method to form a component of an electrical device, for example, a base plate or cover plate of an electrical rotary switch, which is disclosed in U.S. Pat. No. 5,023,414, for detecting positions of an automatic transmission of an automobile, the problem must be settled to prevent the occurrence of electrical problems with the electrical device. As one approach to the problem, it is possible to perform a supplemental step of filling the holes 4M of the molded article 1M with an additional resin 5M. However, an interface formed between the molded article 1M and the additional resin 5M cast in the holes 3M extends from an exterior surface 6M of the molded article 1M to the workpiece 2M provided in its interior surface, as shown in FIG. 12D. Therefore, there is a probability of causing a leakage through such an interface. In particular, when the workpiece 2M is made of a conductive material, the supplemental step would not be enough to provide electrical insulation between the exterior surface 6M of the molded article 1M and the workpiece 2M for an extended time period.
As another approach to the above problem, a molded article 1N can be formed by a double molding process without leaving holes therein. That is, the molding process comprises first and second steps. The first step uses a first mold 10N having lower and upper dies 11N and 12N for making a mold cavity 13N therebetween, supporting pins 14N for supporting a conductive member 2N in the mold cavity 13N, and a gate 15N for injecting an insulating resin 3N into the mold cavity, as shown in FIG. 13A. In the first step, the supporting pins 14N are forced against the conductive member 2M to support the conductive member within the mold cavity 13N, and then the resin 3N is injected into the mold cavity 13N through the gate l5N, as shown in FIG. 13B. The resin injected into the mold cavity 13N is hardened to obtain an intermediate product 4N. The intermediate product 4N has holes 6N left therein by removal of the supporting pins 14N and a plurality of projections 5N, as shown in FIG. 13C.
The second step uses a second mold 20N having lower and upper dies 21N and 22N for making a mold cavity 23N therebetween, and a gate 24N for injecting the insulating resin 3N into the mold cavity 23N, as shown in FIG. 13D. An inner surface of the upper die 22N is forced against the projections 5N of the intermediate product 4N placed on the lower die 21N to support the intermediate product 4N in the mold cavity 23N without using a supporting pin. Then, the resin 3N is injected into the mold cavity 23N through the gate 24N, as shown in FIG. 13E. The resin injected into the mold cavity 23N is hardened to obtain the molded article 1N. Since no supporting pin is used in the second stage, a hole is not left in the molded article 1N. Therefore, it is not necessary to perform such a supplemental step described above.
However, since an interface formed between the intermediate product 4N and the resin 3N injected in the second step extends from an exterior surface 6N of the molded article 1N to the conductive member 2N embedded in its interior surface, as shown in FIG. 13F, there is a probability of causing a leakage through such an interface when the molded article 1N is used for an extended time period. Therefore, the molding process would not be enough to provide electrical insulation between the exterior surface 6N of the molded article 1N and the conductive member 2N provided in its interior surface.